The present invention relates to the field of image scanning devices and in particular to determining ambient light intensity for image scanning devices.
In the imaging industry, scanners are expected to operate effectively under a wide range of ambient light. A number of solutions have been developed wherein ambient light is measured in order to control the scanning system. The sensing of ambient light may be done by an ambient light detection circuit which is separate from the imaging array, or ambient light can be detected through the use of the imaging array itself. The ambient light measurement is then used either to adjust the exposure time for the imaging array/lens, to set the gain of the image signal or to control the brightness of a light source.
U.S. Pat. No. 4,970,379 which issued on Nov. 13, 1990 to Danstrom discloses exposure/illumination control for a bar code scanner consisting of a controllable light source and an optical sensor that is independent of the scanner array. The optical sensor converts the light reflecting from the object to be scanned into an electrical signal representative of the ambient light. This signal is coupled to a comparator, which determines the illumination required by the scanner array, and then adjusts the power to the controllable light source accordingly. A major drawback of this method is that during low light conditions the light source will be driven by the comparator to generate bright illumination, which consumes a large amount of power. In a hand held device this is extremely detrimental, as most hand held devices have a self-contained power supply.
Other systems use the imaging array itself to determine ambient light levels which is then used to control exposure time. U.S. Pat. No. 4,471,228 which issued on Sep. 11, 1984 to Nishizawa et al., describes an image sensor consisting of non-destructive readout-type image cells, the sensor uses the array of image cells as both photo-detector cells for the measurement of ambient light and as image capturing cells for imaging an object. The imaging array is exposed to the object and an ambient light measurement run is made through previously selected imaging cells. The added value of the selected imaging cells is compared to a reference value to determine the exposure level required. The selected imaging cells are then erased, and an image scan of the object is performed with a controlled exposure time.
The shortcoming of this method is that it consists of too many steps. The multi-step process of using the array to measure ambient light and then forcing the array to be reset before the image is scanned, slows the process down. Additionally, the extra step requires an extra expenditure of power, which is a severe detriment in a hand-held device.
Another method using the imaging array is U.S. Pat. No. 4,338,514, which issued on Jul. 6, 1982 to Bixby, discloses a further method of controlling exposure time by operating a mechanical shutter in response to radiant energy impinging on the sensor array. The semiconductor array substrate current is monitored during the exposure of the imaging array to produce an integrated signal that is proportional to the exposure level of the array. The signal is compared to a threshold voltage and when it exceeds a threshold value the shutter is closed.
There are drawbacks to this method in that it requires additional processing steps in order to create an apparatus to monitor the substrate current. Specifically, the apparatus requires the addition of a layer of conductive material between the non-conductive base-plate and the semi-conductive substrate. While this type of process is typical in some CCD imagers, it would be a costly additional fabrication step in a CMOS imager.
A further system in which exposure time is adjusted is described in U.S. Pat. No. 5,986,705 which issued on Nov. 16, 1999 to Shibuya et al. A video camera is described having an image sensing device, an exposure adjustment apparatus which controls the gain of the amplifier to adjust the scanned output signal and further controls a drive pulse generator to control the exposure time of the sensing device. In one embodiment, the video camera controls exposure by capturing an image with the image sensing device, amplifying the output signal which is driven externally as well as being fed back into the exposure adjustment apparatus where the signal is compared to a reference. When the comparison indicates that the image is either overexposed, underexposed or without need of adjustment, control signals are sent to the drive pulse generator to adjust exposure time and to the amplifier to adjust the gain of the amplifier.
This method has several disadvantages, its iterative style of exposure control is only advantageous for a video camera. Controlling only exposure time and signal gain is limiting in terms of the range of light intensity under which the device would remain useful. Still cameras, bar-code readers and the like, would not find such a method useful as it would require additional circuitry to filter out the overexposed and underexposed images. Low-light conditions would be difficult for the device to image as it has no control over an external light source.
While each of the measurement methods has its merits, the measurement methods are inherently limited by either the addition of; extra circuitry, increasing cost and size; fabrication steps, increasing cost; time, slowing the overall performance of the imaging circuit.
Therefore, there is a need for an ambient light detector that is integrated with a scanning device without adding costly additional circuitry and that provides reliable ambient light detection without undue interference with the image capture process.
This invention is directed to a method and apparatus for determining the level of ambient light impinging on a pixel having a photodiode. The method comprises resetting the photodiode in the pixel and at the same time detecting the current flow through the photodiode as an indication of the ambient light level. The photodiode is reset by applying a reverse bias voltage across it and the current flow is detected by measuring the current flow through a resistance in parallel to the photodiode.
In accordance with another aspect, this invention is directed to a method and apparatus for determining the level of ambient light impinging on a selected number of pixels in an imaging array where each pixel includes a photodiode. The ambient light may be determined by resetting the pixels in the array and by detecting current flow through the photodiodes in a selected number of the pixels as they are being reset. Alternately, the ambient light may be determined by resetting a selected number of the pixels in the array and by detecting current flow through the photodiodes in the selected number of the pixels as they are being reset.
The selected number of pixels may be divided into one or more groups each having at least one pixel, and the pixels in each group may be arranged in specific patterns within the array. The array may be laid out in rows and columns, and the groups may be located in predetermined rows or columns. When only a selected number of pixels are reset and these pixels are divided into groups, the groups may be sequentially reset to permit differentiation between the groups.
In accordance with another aspect of this invention, an apparatus determines ambient light on an imaging array of light sensitive pixels where each has a photodiode and photodiode reset switch adapted to apply a predetermined reset voltage across the photodiode and further has one or more power rails each connected to one or more of the pixels for supplying power to them. The apparatus comprises current monitoring circuitry that measures current flow in the photodiodes of selected pixels as the photodiodes are being reset to provide an output signal representative of the ambient light.
With regard to a further aspect of this invention, the current monitoring circuitry and the imaging array may be integrated on the same die.
The current monitoring circuitry may further include one or more current monitors each connected to at least one of the power rails for monitoring the current flow in the photodiodes connected to the power rails and an analog-to-digital converter coupled to each of the current monitors to provide a digital output signal representative of the ambient light. The current monitor may be a current-to-voltage converter connected to a power rail through a resistance. The current-to-voltage converter may include an op-amp having an inverting input terminal coupled to the resistance, a non-inverting input terminal adapted to be coupled to a reference voltage and an output terminal, the output terminal being coupled to the inverting input terminal through a further resistance.
In accordance with an aspect of this invention, in an imaging array where the pixels are positioned in rows and columns, the power rails are may each be connected to a different group of the pixels located in a rows or a column. The power rails for the selected pixels may all be adapted to be connected to the same power supply directly. Alternately, the power rails may each be adapted to be connected to a power supply through a diode or they may each be adapted to be connected to a separate power supply.
With regard to a further aspect of this invention, the apparatus may further include a control for the pixel reset switches that will reset individual groups of pixels sequentially to allow the current in the reset photodiodes to be monitored individually and sequentially.
In accordance with another aspect, this invention may be integrated into a system for controlling the output signal during image capture of an object by an imager where the imager includes an imaging array of light sensitive pixels each having a photodiode and photodiode reset means adapted to apply a predetermined reset voltage across the photodiode, and one or more power rails each connected to one or more pixels on a die.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.